Ensure that the action is backwards-compatible (#25)

Bring 2 changes that ensure that the GitHub Action is backwards compatible:

- Append `http://` if the prefix is not provided in the `OP_CONNECT_HOST` (this is caused by the fact that `curl` guesses the protocol if not provided (https://linux.die.net/man/1/curl), which we missed when switching to using the 1Password CLI as the backend of the action)
- Set the default of export-env to true, since that was the default behavior of the action until we added the possibility to export secrets as step's output.

Also, the documentation is adjusted to reflect these changes.

action.yml

@@ -10,7 +10,7 @@ inputs:
     default: false
   export-env:
     description: Export the secrets as environment variables
-    default: false
+    default: true
 runs:
   using: 'node16'
   main: 'dist/index.js'