Custom partition key name

[kxmas]

I use snake case for json serialization, but the @PartitionKey annotation only uses the java field name. This PR addresses that mismatch.

@PartitionKey("partition_key")
String partitionKey;

[kxmas]

Here is an overview of what got changed by this pull request:

Issues
======
- Added 2

See the complete overview on Codacy

I am unsure as how to satisfy these two Codacy demands. It's complaining about the two small entity classes that I added to the DocumentDbEntityInformationUnitTest in order to properly test the enhancement.

[kxmas]

@Incarnation-p-lee Adding an unused getter and setter fixed the unused field issue. Thanks.

[Incarnation-p-lee]

@kxmas Great, seems the travis CI has some network issue for integration test. I will trigger the build later.

[kxmas]

@kxmas Great, seems the travis CI has some network issue for integration test. I will trigger the build later.

The Travis CI build pipeline appears to be broken. I didn't look too hard, but I can't find a PR where it succeeded.

TestPartitionedPaginationQuery(com.microsoft.azure.spring.data.cosmosdb.core.DocumentDbTemplatePartitionIT)  Time elapsed: 0.261 sec  <<< ERROR!
java.lang.IllegalArgumentException: cosmosdb host key should have text!
    at com.microsoft.azure.spring.data.cosmosdb.core.DocumentDbTemplatePartitionIT.setup(DocumentDbTemplatePartitionIT.java:71)
testPartitionedPaginationQuery(com.microsoft.azure.spring.data.cosmosdb.core.DocumentDbTemplatePartitionIT)  Time elapsed: 0.263 sec  <<< ERROR!
java.lang.NullPointerException
    at com.microsoft.azure.spring.data.cosmosdb.core.DocumentDbTemplatePartitionIT.cleanup(DocumentDbTemplatePartitionIT.java:90)

[kushagraThapar]

@kxmas can you please resolve the conflicts, so I can run additional CI to make sure we are good with all the tests.

[kxmas]

Done

[kushagraThapar]

/azp run

[azure-pipelines[bot]]

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