Unescape encoded Ascii85 payload before decoding

Brief testing shows that when the encoded Ascii85 payload passes
through the Nagios XI (JSON) API that the backslash character is
escaped. Instead of handling this client-side we extend the
decoding process to automatically handle unescaping that
pattern. This also provides a "hook" for further unescaping if
other monitoring systems apply similar escaping of the encoded
payload.

Also, fix various typos in doc comments not caught during
previous polishing passes.

refs GH-251

payload.go

@@ -57,13 +57,31 @@ func EncodeASCII85Payload(data []byte, leftDelimiter string, rightDelimiter stri
 	return leftDelimiter + string(encoded) + rightDelimiter
 }
 
+// unescapeASCII85 unescapes an Ascii85 input payload by removing escape
+// patterns added to the payload as it passes through a monitoring system
+// (e.g., for inclusion in a JSON API response).
+func unescapeASCII85(encodedInput []byte) ([]byte, error) {
+	if len(encodedInput) == 0 {
+		return nil, fmt.Errorf(
+			"failed to unescape empty payload: %w",
+			ErrMissingValue,
+		)
+	}
+
+	// Based on initial testing this is sufficient to unescape an Ascii85
+	// payload that passes through the Nagios XI API.
+	encodedInput = bytes.ReplaceAll(encodedInput, []byte(`\\`), []byte(`\`))
+
+	return encodedInput, nil
+}
+
 // decodeASCII85 decodes given Ascii85 encoded input or an error if one occurs
 // during decoding.
 //
 // The caller is expected to remove any delimiters from the input before
 // calling this function.
 //
-// This function is also not intended for extraction of an Ascii encoded
+// This function is also not intended for extraction of an Ascii85 encoded
 // payload from surrounding text.
 func decodeASCII85(encodedInput []byte) ([]byte, error) {
 	if len(encodedInput) == 0 {
@@ -73,10 +91,15 @@ func decodeASCII85(encodedInput []byte) ([]byte, error) {
 		)
 	}
 
-	decoded := make([]byte, len(encodedInput))
-	n, _, err := ascii85.Decode(decoded, encodedInput, true)
-	if err != nil {
-		return nil, err
+	unescapedInput, unescapeErr := unescapeASCII85(encodedInput)
+	if unescapeErr != nil {
+		return nil, unescapeErr
+	}
+
+	decoded := make([]byte, len(unescapedInput))
+	n, _, decodeErr := ascii85.Decode(decoded, unescapedInput, true)
+	if decodeErr != nil {
+		return nil, decodeErr
 	}
 
 	decodedBytes := decoded[:n]
@@ -94,7 +117,7 @@ func decodeASCII85(encodedInput []byte) ([]byte, error) {
 // occurs during decoding. If provided, the left and right delimiters are
 // trimmed from the given input before decoding is performed.
 //
-// This function is not intended to extract an Ascii encoded payload from
+// This function is not intended to extract an Ascii85 encoded payload from
 // surrounding text.
 func DecodeASCII85Payload(encodedInput []byte, leftDelimiter string, rightDelimiter string) ([]byte, error) {
 	if len(encodedInput) == 0 {