caa records: convert bytes values to string

[freym]

dnspython returns the value and tag for caa records as bytes and octodns tries to set the string representation as values.

Example redacted output from octodns without this change:

********************************************************************************
* example.org.
********************************************************************************
* netbox (NetBoxDNSProvider)
*   Update
*     <CaaRecord CAA 300, example.org., ['0 issue "letsencrypt.org"']> ->
*     <CaaRecord CAA 300, example.org., ['0 b'issue' "b'letsencrypt.org'"']> (netbox)
*   Summary: Creates=0, Updates=1, Deletes=0, Existing Records=1
********************************************************************************

[olofvndrhr]

Looks good to me.
Personally never used it to sync CAA records, so thanks for the fix.