Release 2.1.0-preview.7 (#382)

* Send buffer fix (#380)
* Publish - avoid async state machine when possible (#373)
* Reject payloads over the threshold set by server (#378)
* Fix NatsSvcServer start time format so that it's no longer culture aware (#374)
* Expose Headers on NatsSvcMsg (#371)

Author: mtmk

version.txt

@@ -1 +1 @@
-2.1.0-preview.6
+2.1.0-preview.7