Prior to this commit, the initial buffer size for content caching
allocated in `ContentCachingRequestWrapper` would be:
* the request content length, if available in request headers
* the cache limit size as configured on the wrapper
The latter is really an upper bound and should not be considered as a
good default in most cases. This commit ensures that the request content
length is still used if available, but uses a default 1024 size if it's
not.
While this change will probably cause more reallocations as the buffer
grows, this will avoid large allocations in many cases and should
overall help with GC.
Closes gh-29775
6de0be16c2