Add spec for String#size with encoding change.

The characters number of the string is cached inside
`vm/builtin/string.cpp` in num_char. It should be invalidated
when the encoding changes, for example because of
`String#force_encoding`: with a different encoding, the
correlation between characters and bytes may change completely.
Reset number of characters when changing encoding
Remove tags for passing specs
Merge branch 'razielgn-feature-string-size-encoding-change'