fix(yaml): force quoting when Text values contains a colon char

About
dhall-lang#1939 (comment)

With the current aeson-yaml version used in Dhall, a text value
containing a colon like this one: `Location: https://example.org` is
not quoted in the yaml representation:

```
> dhall-to-yaml <<< '{ headers = ["Location: https://example.org"] }'
headers:
  - Location: https://example.org
```

which is a bug as it doesn't represent the same data.

dhall-yaml/src/Dhall/Yaml.hs

@@ -68,14 +68,15 @@ jsonToYaml json documents quoted =
     style (Y.SStr s)
         | "\n" `Text.isInfixOf` s =
             Right (YE.untagged, YE.Literal YE.Clip YE.IndentAuto, s)
-        | quoted || Text.all isNumberOrDateRelated s || isBoolString =
+        | quoted || Text.all isNumberOrDateRelated s || isBoolString || Text.any isColon s =
             Right (YE.untagged, YE.SingleQuoted, s)
       where
         -- For backwards compatibility with YAML 1.1, we need to add the following to the set of boolean values:
         -- https://yaml.org/type/bool.html
         isBoolString = Text.length s <= 5 &&
                       Text.toLower s `elem` ["y", "yes", "n", "no", "true", "false", "on", "off"]
         isNumberOrDateRelated c = Char.isDigit c || c == '.' || c == 'e' || c == '-'
+        isColon c = c == ':'
     style s =
         YS.schemaEncoderScalar Y.coreSchemaEncoder s