Forced usage of Leaf stringification in line generator.

This allows for overwriting of the stringification method. To be able to
introduce utf-8 version of the PI-character, for example.

line.py

@@ -98,7 +98,7 @@ def generate_line(root):
         return '<empty expression>'
 
     if root.is_leaf:
-        return '-' * root.negated + root.title()
+        return str(root)
 
     content = {}
 
@@ -194,7 +194,7 @@ def generate_line(root):
     # the leafs and nodes in depth first order.
     for node in traverse_depth_first(root):
         if node.is_leaf:
-            content[node] = node.title()
+            content[node] = str(node)
         else:
             arity = len(node)