Optimize xml_buffered_writer::write(const char_t*)

Instead of computing the length and doing memcpy we now copy the head of
the string into the buffer (like strcpy) and then use memcpy for tail if
necessary. This results in 10-15% speedup for writing typical documents with
a mix of short and long strings.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1023 99668b35-9821-0410-8761-19e4c4f06640