Broken dotenv output for values with spaces

Strings containing spaces are not quoted nor escaped in output dotenv files. The only way I've found to produce proper dotenv files in such case is manually quoting each space in the source yaml as quoting isn't possible (related issue #949).

To reproduce

# BAD
$ echo -e "FOO: Some long string with spaces" | sops encrypt --filename-override .yaml | sops decrypt --filename-override .yaml --output-type dotenv
FOO=Some long string with spaces

# BAD
$ echo -e "FOO: 'Some long string with spaces'" | sops encrypt --filename-override .yaml | sops decrypt --filename-override .yaml --output-type dotenv
FOO=Some long string with spaces

# GOOD
$ echo -e "FOO: Some\ long\ string\ with\ spaces" | sops encrypt --filename-override .yaml | sops decrypt --filename-override .yaml --output-type dotenv
FOO=Some\ long\ string\ with\ spaces

Expected behavior

$ echo -e "FOO: Some long string with spaces" | sops encrypt --filename-override .yaml | sops decrypt --filename-override .yaml --output-type dotenv
FOO="Some long string with spaces"

Start at the CLI path for sops decrypt --output-type dotenv and reproduce the provided YAML examples. Done means a value containing spaces is emitted as FOO="Some long string with spaces" while preserving existing dotenv output behavior.