Use correct HEAD request when fetching HTTP headers

[leonardehrenfried]

Summary

When working with large downloads in build-config.json I noticed that starting up OTP was quite slow and using a lot of bandwidth before the graph build, and therefore download, even started.

I traced it down to a problematic method that was supposed to fetch HTTP headers but instead downloaded the entire file.

The Javadoc clearly states that a HEAD request should be executed but the in the acutal implementation this is not the case:

OpenTripPlanner/src/main/java/org/opentripplanner/framework/io/OtpHttpClient.java

Lines 70 to 73 in dc4cdb8

	/**
	* Executes an HTTP HEAD request and returns the headers. Returns an empty list if the HTTP server
	* does not accept HTTP HEAD requests.
	*/

Unit tests

I could write a test spinning up a HTTP server, but I don't think we want that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.78%. Comparing base (e9f847d) to head (60d98e4).
Report is 7 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6063      +/-   ##
=============================================
- Coverage      69.78%   69.78%   -0.01%     
+ Complexity     17358    17355       -3     
=============================================
  Files           1962     1962              
  Lines          74359    74357       -2     
  Branches        7624     7623       -1     
=============================================
- Hits           51895    51890       -5     
- Misses         19821    19822       +1     
- Partials        2643     2645       +2     

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[vpaturet]

Oops, unfortunate mistake in this method, good that you spotted it!