Add fixture methods that strip leading/trailing whitespace

[sleberknight]

We created the Fixtures class because Dropwizard deprecated its FixtureHelpers. Until yesterday I had never seen any differences or problems when changing from FixtureHelpers to Fixtures. But in several services, some tests started failing. The reason is that FixtureHelpers trims leading and trailing whitespace while Fixtures does not. (FixtureHelpers does not mention this in its javadoc, but it clearly calls String#trim in the code.)

This issue is adds one or more methods to Fixtures that strip leading and/or trailing whitespace. Also, since Java 11 added the strip, stripLeading, and stripTrailing methods, these are generally preferred over the (much older) trim method which only handles ASCII whitespace. We could also have methods that use one or the other, but that's probably not necessary right now.

Some example method signatures:

String fixtureStrippingLeadingAndTrailingWhitespace(String resourceName);
String fixtureStrippingLeadingAndTrailingWhitespace(String resourceName, Charset charset);

String fixtureStrippingLeadingWhitespace(String resourceName);
String fixtureStrippingLeadingWhitespace(String resourceName, Charset charset);

String fixtureStrippingTrailingWhitespace(String resourceName);
String fixtureStrippingTrailingWhitespace(String resourceName, Charset charset);

The above names are pretty long, so need to think whether there are shorter names that still convey the intent.

[sleberknight]

In the above method names, I think they are still meaningful, and a tad shorter, by replacing "Stripping" with just "Strip", so fixtureStripLeadingAndTrailingWhitespace

[sleberknight]

The fixtureStripLeadingAndTrailingWhitespace could also omit "LeadingAndTrailing" resulting in fixtureStripWhitespace, which is then similar to the String class, which has strip for removing leading and trailing whitespace. However, leaving it in the method name is probably more clear.