Enforce writing the 11th field name character as null-terminator, incl new tests

Previously field names that exceeded the 10 character limit were capped and written as the first 11 characters, although it should be capped to the first 10 characters and ended with a null-terminator. See #2.

Author: karimbahgat

shapefile.py

@@ -1628,7 +1628,7 @@ def __dbfHeader(self):
             name, fieldType, size, decimal = field
             name = b(name, self.encoding, self.encodingErrors)
             name = name.replace(b' ', b'_')
-            name = name.ljust(11).replace(b' ', b'\x00')
+            name = name[:10].ljust(11).replace(b' ', b'\x00')
             fieldType = b(fieldType, 'ascii')
             size = int(size)
             fld = pack('<11sc4xBB14x', name, fieldType, size, decimal)