[Fix #801] Add `context_dependent` style to BracesAroundHashParameters

Before this change, the no_braces style allowed braces if all
parameters in a method call were hash literals. Looking at the
second last parameter makes more sense. Now we go for more strict
checking, never leaving room for options within one style.
[Fix #1441] Compare AST for whole buffer in AutocorrectUnlessChangingAST

We were already re-parsing the whole buffer to check for syntax
errors. If we check for AST equality instead, we don't need to
check the syntax.