Fix bad request for multipart form data with boundary split (#2159)

* Add test for multipart form data with boundary split

Add a test for multipart form data requests with a large header which
leads to a split inside the boundary because of the read buffer size
inside the SocketStream class.

* Fix bad request for multipart form data with boundary split

Fix a bad request (400) response for multipart form data requests with
a large header which leads to a split inside the boundary because of the
read buffer size inside the SocketStream class. The parse function
inside the MultipartFormDataParser wrongly erase the receive buffer if
it doesn't find the boundary inside the buffer during first call.

httplib.h

@@ -5028,10 +5028,9 @@ public:
     while (buf_size() > 0) {
       switch (state_) {
       case 0: { // Initial boundary
-        buf_erase(buf_find(dash_boundary_crlf_));
-        if (dash_boundary_crlf_.size() > buf_size()) { return true; }
-        if (!buf_start_with(dash_boundary_crlf_)) { return false; }
-        buf_erase(dash_boundary_crlf_.size());
+        auto pos = buf_find(dash_boundary_crlf_);
+        if (pos == buf_size()) { return true; }
+        buf_erase(pos + dash_boundary_crlf_.size());
         state_ = 1;
         break;
       }