gh-150751: Ignore optional whitespace around Content-Length in http.client (GH-159156)

gh-150752 started validating the Content-Length value against the
RFC 9112 grammar (1*DIGIT), but the compat32 header parser keeps
trailing SP/HTAB in the field value, so "Content-Length: 5 " is now
treated as a missing header. HTTPResponse then assumes that the
connection will close and read() blocks until the server closes it,
which never happens on a keep-alive connection.

RFC 9112, section 5.1 says that optional whitespace around a field
value is excluded by parsers. Strip SP/HTAB before the grammar check;
values such as "+5", "5_0" and "5 0" are still rejected.