Provide response text passthrough for GPX-formatted requests

[jayaddison]

Resolves #43.

[codecov-commenter]

Codecov Report

Merging #67 (834e959) into master (ed5e5cd) will decrease coverage by 0.37%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##            master      #67      +/-   ##
===========================================
- Coverage   100.00%   99.62%   -0.38%     
===========================================
  Files           12       12              
  Lines          525      530       +5     
===========================================
+ Hits           525      528       +3     
- Misses           0        2       +2     

Flag	Coverage Δ
unittests	99.62% <71.42%> (-0.38%)	⬇️

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Impacted Files	Coverage Δ
openrouteservice/directions.py	100.00% <ø> (ø)
openrouteservice/client.py	98.40% <71.42%> (-1.60%)	⬇️

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[jayaddison]

cc @koebi @MichaelsJP (not urgent; I noticed issue #67 a while ago and figured it'd be worth attempting an implementation)

[MichaelsJP]

@jayaddison Thx a lot for the contribution. 👌 I'll have a look at it the next days.

[jayaddison]

Thanks @MichaelsJP!