Unsound use of String::as_mut_vec

[mzabaluev]

As noted in a review for a proposed change, the merge implementation for strings breaks the UTF-8 invariant of a String value modified in place, in case when wire data being merged are not valid UTF-8.

[mzabaluev]

To fix this, I need would like to understand if there is a reason why bytes::merge and string::merge do not clear the field value before extending it with the wire data. My reading of the protobuf spec suggests that the implementation should replace the value of a non-repeating field if it's seen in the same message multiple times, but bytes::merge appends to the value instead.