Force traces in stream to first dtype before merge

A Stream corresponding to a given network, station, location, and
channel can (but probably shouldn't) have more than 1 trace returned
from an ObsPy client.get_waveforms() function. This happens, for
example, when the data type generated by a given station changes.
This change forces the dtype of the data array for all Traces in
the Stream to match the dtype of the data array in the first Trace
of the Stream, thus allowing the Stream.merge() function to work
as intended.