ENH: Support multiline header fields in TCK

Closes gh-957

Author: effigies

nibabel/streamlines/tck.py

@@ -331,23 +331,30 @@ def _read_header(cls, fileobj):
             f.seek(1, os.SEEK_CUR)  # Skip \n
 
             found_end = False
+            key = None
+            lines = []
 
             # Read all key-value pairs contained in the header, stop at EOF
             for n_line, line in enumerate(f, 1):
-                line = line.decode('utf-8').strip()
-
-                if not line:  # Skip empty lines
-                    continue
-
-                if line == 'END':  # End of the header
+                if line.strip() == b'END':
                     found_end = True
-                    break
 
-                if ':' not in line:  # Invalid header line
-                    raise HeaderError(f'Invalid header (line {n_line}): {line}')
-
-                key, value = line.split(':', 1)
-                hdr[key.strip()] = value.strip()
+                # Multiline values mean we don't know we're done until we find the next value
+                if found_end or b':' in line:
+                    if key is not None:
+                        hdr[key] = b''.join(lines).decode().strip()
+                    if found_end:
+                        break
+
+                    parts = line.split(b':', 1)
+                    key = parts[0].decode().strip()
+                    lines = [parts[1]]
+                elif key is None:
+                    # Apparent continuation line before any keys are found
+                    if line.strip():
+                        raise HeaderError(f'Invalid header (line in {n_line}): {line}')
+                else:
+                    lines.append(line)
 
             if not found_end:
                 raise HeaderError('Missing END in the header.')