Add support for SpEL in @Query

[puppylpg]

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Closes #2083

[sothawo]

Nice work. I have some remarks and questions and will do more tests later that week (being a little busy currently in the evenings).

And this needs to be added to the reactive parts as well.

[sothawo]

quoted elements means Strings? What about other types, i.e. numeric ones?

[puppylpg]

The non string types will be converted correctly by other converters in ConversionSerivce. Take integers as the example:

				{
				  "bool":{
				    "must":{
				      "terms":{
				        "age": [1, 2, 3]
				      }
				    }
				  }
				}

I'll add more tests to prove this.

[puppylpg]

And the comment is also changed to avoid this misleading.

[sothawo]

targetElement can be null at this place; the + and the valueOf will convert this to `"null"; is that ok or should that produce an error?

[puppylpg]

I've added the code to check that source is not null, and target may have no need to check null again after that.

[puppylpg]

Review suggestions are accepted and more tests has been added. Reactive parts will be handle later.

UPDATE: reactive parts is also finished.