GH-3121 Throw PropertyReferenceException for whitespace-starting Prop…

[mipo256]

Closes #3121

So, actually, the situation that is described in the original ticket is quite flawed in many ways.

My basic assumption is that PropertyPath cannot hold whitespace holding properties. Now, the way it works with properties. key as the PropertyPath in spring data mongodb - it actually don't, we still were unable to parse the PropertyPath, but we handed the exception - the PropertyReferenceException.

The mongodb can handles property references like properties. key, so the spring-data-mongodb just passed it literally as

{ $set : { "properties. key" : "value" }  }

and the mongodb engine would figure it out.

Therefore, I think, that the only correct fix that can be applied here is to just throw a correct exception, which is PropertyReferenceException. The properties. key and properties. Key should not be considered valid PropertyPath instances.

CC: @christophstrobl @mp911de

[mipo256]

I'll add the Integration test for mongodb to make sure that this case works