Fix get set reversibility (#158)

* prefix keys with mock-redis-test to ensure cleared between runs

* test for string conversion in get method

lib/mock_redis/string_methods.rb

@@ -85,6 +85,7 @@ def decrby(key, n)
     end
 
     def get(key)
+      key = key.to_s
       assert_stringy(key)
       data[key]
     end

spec/commands/get_spec.rb

@@ -24,6 +24,7 @@
 
     @redises.set(key, 'hello')
     @redises.get(key.to_s).should == 'hello'
+    @redises.get(key).should == 'hello'
   end
 
   it_should_behave_like 'a string-only command'

spec/commands/xadd_spec.rb

@@ -49,7 +49,7 @@
   end
 
   it 'raises exception if id of 0 is added to an empty stream' do
-    expect { @redises.xadd('unknown-stream', { key: 'value' }, id: '0') }
+    expect { @redises.xadd('mock-redis-test:unknown-stream', { key: 'value' }, id: '0') }
       .to raise_error(
         Redis::CommandError,
         'ERR The ID specified in XADD is equal or smaller than the target ' \
@@ -58,7 +58,7 @@
   end
 
   it 'does not raise exception on empty stream with id of 0 and positive sequence number' do
-    expect { @redises.xadd('unknown-stream', { key: 'value' }, id: '0-1') }
+    expect { @redises.xadd('mock-redis-test:unknown-stream', { key: 'value' }, id: '0-1') }
       .to_not raise_error
   end