Fix rare memory leak in XML Error copy-assignment

[SergeyRyabinin]

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:

• Did a review by yourself.
• 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.

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[SergeyRyabinin]

this was missing, the rest are just nitty decorations.