Parser: fix the condition to decide var or call

[makenowjust]

There is source code foo, this foo is decided var or call on parsing, and the condition for this has some bugs.

First bug, foo(+1) is parsed as foo + 1 if foo is variable. For example:

foo = 1
p foo(+1) # => 2 (wtf??)

Second bug, case 1 when foo then 2 end is parsed as case 1 when foo() then 2 end even if foo is variable. For example:

foo = 1
case 1
when foo then 2
end

# Error: undefined method 'foo'

These bugs are fixed in this PR. Thank you.