Fix key type restriction of Hash#merge block

[makenowjust]

The key type restriction of Hash#merge block is wrong. K (receiver's key type) is specified, but it yields L (argument's key type) value in fact. This type mismatch causes a problem when key type is difference. For instance, the following sample cannot work due to this bug.

foo = {1 => "foo"}
bar = {1 => "bar", "fizz" => "buzz"}

p foo.merge(bar) { |k, v1, v2| v1 + v2 }
# Error: argument #1 of yield expected to be Int32, not (Int32 | String)

This PR fixes this restriction type into L correctly.

[jhass]

Mmh, does it also fix

foo = {1 => "foo"}
bar = {"fizz" => "buzz"}

p foo.merge(bar) { |k, v1, v2| v1 + v2 }

EDIT: Nvm, it does :)

[straight-shoota]

Thank you @makenowjust