Strip leading space when wrapped lines are joined.

From RFC 5545:

"a long line can be split between any two characters by inserting a CRLF
immediately followed by a single linear white-space character (i.e.,
SPACE or HTAB).  Any sequence of CRLF followed immediately by a
single linear white-space character is ignored (i.e., removed) when
processing the content type."

http://tools.ietf.org/html/rfc5545#section-3.1