fix: use header name matching trace state specs (#114)

However it seems that current implementation does slightly different
thing than the [specification][1].

[1]: https://github.com/w3c/trace-context/blob/master/spec/20-HTTP_HEADER_FORMAT.md#tracestate-field

Author: hauleth

src/oc_tag_ctx_header.erl

@@ -25,7 +25,7 @@
 -include("opencensus.hrl").
 
 field_name() ->
-    <<"Trace-State">>.
+    <<"tracestate">>.
 
 -spec encode(oc_tags:tags()) -> maybe(iodata()).
 encode(Tags) when map_size(Tags) =:= 0 ->