[Varnish] #1598: Varnish asserts if the backend returns header lines without ':' and doing backend IMS

[Varnish]

#1598: Varnish asserts if the backend returns header lines without ':' and doing
backend IMS
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 Reporter:  martin    |      Owner:
     Type:  defect    |     Status:  new
 Priority:  normal    |  Milestone:
Component:  varnishd  |    Version:  trunk
 Severity:  normal    |   Keywords:
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 See attached test case.

 The test case fails both current master and the 4.0 branch. Asserts on not
 finding the char ':' in the string in HTTP_Merge().

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1598>
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