Prior to this commit, one could write a `CharSequence` to an existing
`DataBuffer` instance by turning it into a byte array or `ByteBuffer`
first. This had the following disadvantages:
1. Memory allocation was not efficient (not leveraging pooled memory
when available)
2. Dealing with `CharsetEncoder` is not always easy
3. `DataBuffer` implementations, like `NettyDataBuffer` can use
optimized implementations in some cases
This commit adds a new `DataBuffer#write(CharSequence, Charset)` method
for those cases and also an `ensureCapacity` method useful for checking
that the current buffer has enough capacity to write to it..
Issue: SPR-17558
6361b0cb23