Gets the header names of this Part.
<p>Some servlet containers do not allow servlets to access headers using this method, in which case this method returns <code>null</code>
<p>Any changes to the returned <code>Collection</code> must not affect this <code>Part</code>.
@return a (possibly empty) <code>Collection</code> of the header names of this Part
See Implementation
Gets the header names of this Part.
<p>Some servlet containers do not allow servlets to access headers using this method, in which case this method returns <code>null</code>
<p>Any changes to the returned <code>Collection</code> must not affect this <code>Part</code>.
@return a (possibly empty) <code>Collection</code> of the header names of this Part