[http] Accept Content-Length header with trailing whitespace

At least one HTTP server (Google's OCSP responder) has been observed
to generate a Content-Length header with trailing whitespace.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpcore.c

 #include <strings.h>
 #include <byteswap.h>
 #include <errno.h>
+#include <ctype.h>
 #include <assert.h>
 #include <ipxe/uri.h>
 #include <ipxe/refcnt.h>
 
 	/* Parse content length */
 	content_len = strtoul ( value, &endp, 10 );
-	if ( *endp != '\0' ) {
+	if ( ! ( ( *endp == '\0' ) || isspace ( *endp ) ) ) {
 		DBGC ( http, "HTTP %p invalid Content-Length \"%s\"\n",
 		       http, value );
 		return -EINVAL_CONTENT_LENGTH;