Fix rare memory leak in XML Error copy-assignment

Issue #, if available: rare memory leak in such conditions:

    AWSError<CoreErrors> error = awsErrorMarshaller.Marshall(*BuildHttpXmlResponse("IncompleteSignatureException", message, requestId, IllFormed));
    AWSError<CoreErrors> emptyError;
    error = emptyError; // i.e. assign empty AWSError to a non-empty XML AWSError

Description of changes: add Aws::Delete to

XmlDocument& XmlDocument::operator=(const XmlDocument& other)
<...>
Aws::Delete(m_doc);

Check all that applies:

[x] Did a review by yourself.
[x] Added proper tests to cover this PR. (If tests are not applicable, explain.)
[ ] Checked if this PR is a breaking (APIs have been changed) change.
[ ] Checked if this PR will not introduce cross-platform inconsistent behavior.
[ ] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

[x] Linux
[x] Windows
[ ] Android
[ ] MacOS
[ ] IOS
[ ] Other Platforms

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aws / aws-sdk-cpp

Fix rare memory leak in XML Error copy-assignment #2905