TSMimeHdrFieldValueStringSet() sets the value of a MIME field. The field is identified by the
combination of bufp, hdr, and field which should match those passed to the
function that returned field such as
TSMimeHdrFieldFind(). The value is copied
to the header represented by bufp. value does not have to be null terminated (and in
general should not be).
If idx is non-negative the existing value in the field is treated as a multi-value and
idx as the 0 based index of which element to set. For example if the field had the value
dave, grigor, tamara and
TSMimeHdrFieldValueStringSet() was called with value of
syeda and idx of 1, the value would be set to
dave, syeda, tamara. If idx is
non-negative it must be the index of an existing element or exactly one past the last element or the
call will fail. In the example case idx must be between
TSMimeHdrFieldValuesCount() can be used to get the current number of elements.