output/httpd: flush encoder after tag

Without the flush, ReadPage() may not return any data, or not all
data.  This may result in incomplete ddata the new "header" page,
corrupting streams with some encoders such as Vorbis.

Fixes #145

NEWS

@@ -12,6 +12,7 @@ ver 0.20.12 (not yet released)
   - vorbis: default to quality 3
 * output
   - fix hanging playback with soxr resampler
+  - httpd: flush encoder after tag; fixes corrupt Vorbis stream
 
 ver 0.20.11 (2017/10/18)
 * storage

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -468,6 +468,7 @@ HttpdOutput::SendTag(const Tag &tag)
 
 		try {
 			encoder->SendTag(tag);
+			encoder->Flush();
 		} catch (const std::runtime_error &) {
 			/* ignore */
 		}