Ruby: extend mixin field

[AmaiKinono]

The solution developed in #2478 is extended to deal with "prepend" and "extend" methods.

One thing I've noticed is include, prepend and extend are all methods, so it's valid to write:

class A
  prepend(X)
end

But it's not handled by current scheme.

[coveralls]

Coverage increased (+0.001%) to 86.71% when pulling db63ee3 on AmaiKinono:ruby-mixin into 6c50aae on universal-ctags:master.

[codecov]

Codecov Report

Merging #2481 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2481   +/-   ##
=======================================
  Coverage   86.60%   86.60%           
=======================================
  Files         177      177           
  Lines       36759    36763    +4     
=======================================
+ Hits        31835    31839    +4     
  Misses       4924     4924           

Impacted Files	Coverage Δ
parsers/ruby.c	98.03% <100.00%> (+0.03%)	⬆️

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[masatake]

Thank you!

[masatake]

class A
  prepend(X)
end

This can be captured with:

[yamato@slave]~/var/ctags-github% git diff parsers/ruby.c
diff --git a/parsers/ruby.c b/parsers/ruby.c
index f6ac449c..b6d465b2 100644
--- a/parsers/ruby.c
+++ b/parsers/ruby.c
@@ -476,8 +476,13 @@ static void readAndStoreMixinSpec (const unsigned char** cp, const char *how_mix
    if (! (e->kindIndex == K_CLASS || e->kindIndex == K_MODULE))
        return;

-   if (isspace (**cp))
+   if (isspace (**cp) || (**cp == '('))
    {
+       if (isspace (**cp))
+           skipWhitespace (cp);
+       if (**cp == '(')
+           ++*cp;
+
        vString *spec = vStringNewInit (how_mixin);
        vStringPut(spec, ':');

I will make a pull request for this after merging #2482.