Make sure string filtering is case insensitive for WFS requests

[asaunier]

Related to #2855

It seems there are no unit tests for the RuleHelper service and I see no other test that could be related.

Documentation of the ol.format.filter.like function, whose arguments are edited in this PR to explicitly add matchCase=false (default value is undefined => this attribute didn't appear previously in the WFS request):
http://openlayers.org/en/latest/apidoc/ol.format.filter.html#.like

Posted WFS GetFeature request formerly contained:

<PropertyIsLike wildCard="*" singleChar="." escapeChar="!"><PropertyName>name</PropertyName><Literal>linde</Literal></PropertyIsLike>

Now is:

<PropertyIsLike wildCard="*" singleChar="." escapeChar="!" matchCase="false"><PropertyName>name</PropertyName><Literal>linde</Literal></PropertyIsLike>

=> returns results whose name are eg. "Linde", "Lindenpark", ....

[asaunier]

@sbrunner Thanks for reviewing: merging...

[asaunier]

@sbrunner My PR was against the master branch. Is it OK or should it be pushed to ngeo 2.2 instead?