Fail gracefully on SPLASH_URL without protocol

[Gallaecio]

Fixes #120

[codecov]

Codecov Report

Merging #214 into master will increase coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   92.67%   93.05%   +0.37%     
==========================================
  Files           9        9              
  Lines         587      590       +3     
  Branches      118      119       +1     
==========================================
+ Hits          544      549       +5     
+ Misses         21       20       -1     
+ Partials       22       21       -1

Impacted Files	Coverage Δ
scrapy_splash/middleware.py	90.56% <100%> (+0.87%)	⬆️

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