Send headers at connection for HTTP Server observe property

[relu91]

Fix #1065. We are now using the same policy that we used for events.

Update: About tests, the current test suite is pretty messy. If you have ideas on how to start testing and better organizing the HTTP server feel free to write a proposal.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.05% ⚠️

Comparison is base (7c00309) 75.38% compared to head (61c6ecb) 75.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1067      +/-   ##
==========================================
- Coverage   75.38%   75.33%   -0.05%     
==========================================
  Files          80       80              
  Lines       15443    15452       +9     
  Branches     1477     1477              
==========================================
  Hits        11641    11641              
- Misses       3766     3775       +9     
  Partials       36       36              

Files Changed	Coverage Δ
...ckages/binding-http/src/routes/property-observe.ts	18.34% <0.00%> (-1.66%)	⬇️

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[danielpeintner]

As @relu91 mentioned it matches with what we do for events

node-wot/packages/binding-http/src/routes/event.ts

Lines 68 to 75 in 7c00309

	if (!res.headersSent) {
	// We are polite and use the same request as long as the client
	// does not close the connection (or we hit the timeout; see below).
	// Therefore we are sending the headers
	// only if we didn't have sent them before.
	res.setHeader("Content-Type", value.type);
	res.writeHead(200);
	}