Add autocorrect to Rails/AssertEmptyLiteral cop

[fatkodima]

1. Added autocorrection
2. Also this cop was not implemented correctly as it should check against assert_equal([], something), not assert([], something) - changed that

For reference: http://docs.seattlerb.org/minitest/Minitest/Assertions.html#method-i-assert

[fatkodima]

Rebased.

[koic]

1. Added autocorrection
2. Also this cop was not implemented correctly as it should check against assert_equal([], something), not assert([], something) - changed that

I think that unsafe auto-correction could be added. But the assert method has been intentionally used.
#32

I'm worried about changing behaviour for bad case.

[fatkodima]

But the assert method has been intentionally used. #32

Aaaah, got it.

Reverted the second part:

Also this cop was not implemented correctly as it should check against assert_equal([], something), not assert([], something) - changed that

Marked unsafe autocorrection.

[koic]

Thank you!