Support for standard type union discriminant with constant value (#44)

`Union.resolved_case` doesn't support unions with standard type
discriminant and constant (`const`) values, ex:

```xdr
enum StellarValueType
{
    STELLAR_VALUE_BASIC = 0,
    STELLAR_VALUE_SIGNED = 1
};

union switch (int v)
{
case STELLAR_VALUE_BASIC:
    void;
    ...
```

To fix this, we are searching for identifier in namespace's enum
constants if discriminant type has not been found.

lib/xdrgen/ast/definitions/union.rb

@@ -19,8 +19,27 @@ class Union < Base
       end
 
       def resolved_case(kase)
-        found = discriminant_type.members.find{|m| m.name == kase.value_s}
-        raise "Case error:  #{kase} is not a member of #{discriminant_type.name}" if found.nil?
+        if discriminant_type.nil? then
+          # discriminant_type has not been found we need to search for the value in namespace's enum constants.
+          # It's a case where union discriminant is a standard type (like `int`):
+          #
+          # enum StellarValueType
+          # {
+          #     STELLAR_VALUE_BASIC = 0,
+          #     STELLAR_VALUE_SIGNED = 1
+          # };
+          #
+          # union switch (int v)
+          # {
+          # case STELLAR_VALUE_BASIC:
+          #     void;
+          #     ...
+          found = namespace.find_enum_value(kase.value_s)
+          raise "Case error:  #{kase} (#{kase.value_s}) constant not found" if found.nil?
+        else
+          found = discriminant_type.members.find{|m| m.name == kase.value_s}
+          raise "Case error:  #{kase} is not a member of #{discriminant_type.name}" if found.nil?
+        end
         found
       end
 

lib/xdrgen/generators/go.rb

@@ -534,7 +534,11 @@ def switch_for(out, union, ident)
 
             value = if c.value.is_a?(AST::Identifier)
                       member = union.resolved_case(c)
-                      "#{name union.discriminant_type}#{name member}"
+                      if union.discriminant_type.nil? then
+                        "int32(#{name member.enum}#{name member})"
+                      else
+                        "#{name union.discriminant_type}#{name member}"
+                      end
                     else
                       c.value.text_value
                     end

lib/xdrgen/generators/java.rb

@@ -251,7 +251,12 @@ def render_union(union, out)
             else
               arm.cases.each do |kase|
                 if kase.value.is_a?(AST::Identifier)
-                  out.puts "case #{kase.value.name}:"
+                  if type_string(union.discriminant.type) == "Integer"
+                    member = union.resolved_case(kase)
+                    out.puts "case #{member.value}:"
+                  else
+                    out.puts "case #{kase.value.name}:"
+                  end
                 else
                   out.puts "case #{kase.value.value}:"
                 end
@@ -285,7 +290,12 @@ def render_union(union, out)
             else
               arm.cases.each do |kase|
                 if kase.value.is_a?(AST::Identifier)
-                  out.puts "case #{kase.value.name}:"
+                  if type_string(union.discriminant.type) == "Integer"
+                    member = union.resolved_case(kase)
+                    out.puts "case #{member.value}:"
+                  else
+                    out.puts "case #{kase.value.name}:"
+                  end
                 else
                   out.puts "case #{kase.value.value}:"
                 end

lib/xdrgen/generators/javascript.rb

@@ -128,7 +128,12 @@ def render_union(out, union)
 
               arm.cases.each do |acase|
                 switch = if acase.value.is_a?(AST::Identifier)
-                  '"' + member_name(acase.value) + '"'
+                  if union.discriminant.type.is_a?(AST::Typespecs::Int)
+                    member = union.resolved_case(acase)
+                    "#{member.value}"
+                  else
+                    '"' + member_name(acase.value) + '"'
+                  end
                 else
                   acase.value.text_value
                 end