According to RFC 2616 [1] header field names are case insensitive.
Header fields defined as a comma separated list can have multiple header
fields with the same field name. Add a method to HttpConnection which
retrieves all values with a given header field name with the field name
compared case insensitive.
[1] https://tools.ietf.org/html/rfc2616#section-4.2"
Change-Id: I7f601b21cda99e84f43f866c7c7cb4cb0e3cf5c3
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>