Fix handling hash rocket token after symbol

The method what creates TkIDENTIFIER, identify_identifier,
it takes continuing `=` for method name as symbol
because `RDoc::RubyLex` handles some symbols
via TkSYMBEG and TkIDENTIFIER.
But it is sometimes broken for hash literal, like below:

{ :method_name => :value } # keys is :method_name, success
{ :method_name= => :value } # key is :method_name=, success
{ :method_name=> :value } # key is :method_name, failure

This commit fixes the behaviour.

Author: aycabta

lib/rdoc/ruby_lex.rb

@@ -966,7 +966,7 @@ def identify_identifier
 
     if @lex_state == :EXPR_FNAME
       @lex_state = :EXPR_END
-      if peek(0) == '='
+      if peek(0) == '=' and peek(1) != '>'
         token.concat getc
       end
     elsif @lex_state == :EXPR_BEG || @lex_state == :EXPR_DOT ||