Remove spaces in SLD exports for multipart expressions

[geographika]

A case that was missed in #6983 - multipart expressions such as the following:

EXPRESSION ( ( [FID] > 1 ) AND ( [FID] <= 10 ) )

Were still retaining spaces:

<ogc:And><ogc:PropertyIsGreaterThan><ogc:PropertyName>FID</ogc:PropertyName><ogc:Literal>1 </ogc:Literal>
</ogc:PropertyIsGreaterThan><ogc:PropertyIsLessThanOrEqualTo><ogc:PropertyName>FID</ogc:PropertyName>
<ogc:Literal>10 </ogc:Literal></ogc:PropertyIsLessThanOrEqualTo></ogc:And>

This change removes the spaces. A test also added to ensure that strings with spaces e.g. "my value " retain the trailing space.