Detect and trim any invalid incoming req header name

[dino-brzika]

Issue #1261
In web-incoming.js it is not handled if incoming req header name start or end with whitespace.
Result is crashing of application. Issue is easy to reproduce by manually adding in req headers header with whitespace (example 'cookie '). In production issue is observed on node 4.8.0 and http-proxy 1.16.2 version. Application is build in Meteor which does not handle this as well.
Submitted solution is to loop through req headers and when invalid header is detected, add new header with trimmed name and remove existing invalid header. In addition to this it will replace remaining whitespaces with dash(-). This is needed in order to handle case where header is named like "foo bar".
Fix verified on node 4.8.0 and all tests passed.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a3fe02d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1276   +/-   ##
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  Coverage          ?   92.54%           
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  Files             ?        6           
  Lines             ?      322           
  Branches          ?        0           
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  Hits              ?      298           
  Misses            ?       24           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/http-proxy/passes/web-incoming.js	98.52% <100%> (ø)

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