fix field termination for terminators longer than one byte

atest/terminated_fields.txt

@@ -5,26 +5,28 @@ Suite teardown    Reset Rammbock
 Resource    Protocols.txt
 Default tags    Regression
 
-*** Variables ***
-${default terminator}=    0x00
-
-
 *** Test Cases ***
 Get message with chars and end byte
-    Message with chars and end byte
+    Message with chars and end byte    0x00
     ${msg}    get message    arbitrary_length:foo.bar.zig
     Should be equal    foo.bar.zig    ${msg.arbitrary_length.ascii}
     should be equal as integers   12    ${msg.arbitrary_length.len}
 
+Get message with chars and end byte long termination
+    Message with chars and end byte    0x0d0a
+    ${msg}    get message    arbitrary_length:foo.bar.zig
+    Should be equal    foo.bar.zig    ${msg.arbitrary_length.ascii}
+    Should be equal    foo.bar.zig\r\n    ${msg.arbitrary_length._raw}
+    should be equal as integers   13    ${msg.arbitrary_length.len}
+
 Send and receive message with chars with end byte
     [tags]
     Setup TCP server and client     Test
-    Message with chars and end byte
+    Message with chars and end byte      0x0d0a
     Client sends message    arbitrary_length:foo.bar.zig
     ${msg}=    Server receives message
     Should be equal    foo.bar.zig    ${msg.arbitrary_length.ascii}
-    should be equal as integers   12    ${msg.arbitrary_length.len}
-
+    should be equal as integers   13    ${msg.arbitrary_length.len}
 
 *** Keywords ***
 Define protocol
@@ -34,6 +36,7 @@ Define protocol
     end protocol
 
 Message with chars and end byte
+    [Arguments]    ${terminator}
     new message    ExampleMessage    Test
-    chars    *    arbitrary_length    ${none}    terminator=${default terminator}
+    chars    *    arbitrary_length    ${none}    terminator=${terminator}
     uint    8     some_integer    0x00

src/Rammbock/templates/primitives.py

@@ -207,7 +207,7 @@ def _encode_value(self, value, message, little_endian=False):
 
     def _prepare_data(self, data):
         if self._terminator:
-            return data[0:data.index(self._terminator) + 1]
+            return data[0:data.index(self._terminator) + len(self._terminator)]
         return data