/series/add: don't try to re-display file upload field.

Before this change upload field re-displayed as:
<input name="image" type="file" value="org.springframework.web.multipart.support.StandardMultipartHttpServletRequest$StandardMultipartFile@7a96cce0"/>
/series/add: fixed re-displaying category and country when form input was invalid.

Also made tests less brittle.
Raised log level for message about entities creation.

Corrections for the following commits:
- 7b43bad8b0c90b8ca2c7f8203b53c97aff7afdb8 (CountryServiceImpl and SeriesServiceImpl)
- 54c9ab9a899c1afbc5f7021f2c8db55a34b6a27d (CategoryServiceImpl)

No functional changes.