Previously we used the setters and getters in the CopyFrom method. The
getters and setters both caps a signal to be within [min, max] range.
This meant that if we used CopyFrom from a message which contained an
out-of-bounds signal the new value would not keep the same signal
value.
This commit updates the CopyFrom implementation to instead unmarshal
the generated CAN frame from the original message.
BREAKING CHANGE: This changes CopyFrom to accept a message instead of
a message reader to copy from, however mostly test code should be
affected.
