WebDAV: send "User-Agent: SyncEvolution"

Apple iCloud servers reject requests unless they contain a User-Agent
header. The exact value doesn't seem to matter. Making the string
configurable might be better, but can still be done later when it
is more certain whether and for what it is needed.

src/backends/webdav/NeonCXX.cpp

@@ -346,6 +346,12 @@ void Session::preSend(ne_request *req, ne_buffer *header)
         SE_THROW("internal error: startOperation() not called");
     }
 
+    bool haveUserAgentHeader = boost::starts_with(header->data, "User-Agent:") ||
+        strstr(header->data, "\nUser-Agent:");
+    if (!haveUserAgentHeader) {
+        ne_buffer_concat(header, "User-Agent: SyncEvolution\r\n", (const char *)NULL);
+    }
+
     // Only do this once when using normal username/password.
     // Always do it when using OAuth2.
     bool useOAuth2 = m_authProvider && m_authProvider->methodIsSupported(AuthProvider::AUTH_METHOD_OAUTH2);