Negative gain for ZWO cameras does not populate into file name.

[Kristopher McGinnis]

Using the camera pattern $$GAIN$$ with a negative gain set for ZWO cameras results in no value being produced in the filename.

Steps to reproduce:
Set the camera gain to a negative number.
Have the image file pattern include the $$GAIN$$ pattern.
Take an image and look at the filename.

Expected behavior: the filename will include the gain setting.
Actual behavior: the filename does not include the gain setting (if set to a negative value).

[Stefan B]

Duplicate of #1286.

[Stefan B]

Negative gain is currently not supported.