Add support for US-ASCII (7bit) encoding

Implemented a new encoding type, `EncodingUSASCII`, representing US-ASCII (7bit) encoding. Updated the test suite to include this new encoding in the test cases, ensuring proper validation and coverage.

encoding.go

@@ -27,6 +27,9 @@ const (
 	// EncodingQP represents the "quoted-printable" encoding as specified in RFC 2045.
 	EncodingQP Encoding = "quoted-printable"
 
+	// EncodingUSASCII represents encoding with only US-ASCII characters (aka 7Bit)
+	EncodingUSASCII Encoding = "7bit"
+
 	// NoEncoding avoids any character encoding (except of the mail headers)
 	NoEncoding Encoding = "8bit"
 )

encoding_test.go

@@ -16,6 +16,7 @@ func TestEncoding_String(t *testing.T) {
 		{"Encoding: Base64", EncodingB64, "base64"},
 		{"Encoding: QP", EncodingQP, "quoted-printable"},
 		{"Encoding: None/8bit", NoEncoding, "8bit"},
+		{"Encoding: US-ASCII/7bit", EncodingUSASCII, "7bit"},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {