Do not wrap headers in markdown.tpl because inconsistency in the interpretation of multiline headers.

IPython/nbconvert/templates/markdown.tpl

@@ -1,73 +1,73 b''
 {% extends 'display_priority.tpl' %}
 {% block in_prompt %}
 In[{{cell.prompt_number if cell.prompt_number else ' '}}]:{% endblock in_prompt %}
 
 {% block output_prompt %}{% if cell.haspyout %}Out[{{cell.prompt_number}}]:
 {%- endif %}{%- endblock output_prompt %}
 
 {% block input %}
 ```
 {{ cell.input}}
 ```
 {% endblock input %}
 
 {% block pyerr %}
 {{ super() }}
 {% endblock pyerr %}
 
 {% block traceback_line %}
 {{ line |indent| rm_ansi }}{% endblock traceback_line %}
 
 {% block pyout %}
 {% block data_priority scoped %}{{ super()}}{% endblock %}
 {% endblock pyout %}
 
 {% block stream %}
 {{ output.text| indent }}
 {% endblock stream %}
 
 
 
 
 {% block data_svg %}
 [!image]({{output.key_svg}})
 {% endblock data_svg %}
 
 {% block data_png %}
 [!image]({{output.key_png}})
 {% endblock data_png %}
 
 {% block data_jpg %}
 [!image]({{output.key_jpg}})
 {% endblock data_jpg %}
 
 
 
 {% block data_latex %}
 $$
 {{output.latex}}
 $$
 {% endblock data_latex %}
 
 {% block data_text scoped %}
 
 {{output.text | indent}}
 
 {% endblock data_text %}
 
 {% block markdowncell scoped %}
 {{ cell.source | wrap(80)}}
 {% endblock markdowncell %}
 
 {% block headingcell scoped %}
 
-{{ '#' * cell.level }} {{ cell.source | wordwrap(80, False, '\n'+'#' * cell.level+' ')}}
+{{ '#' * cell.level }} {{ cell.source }}
 
 {% endblock headingcell %}
 
 {% block rawcell scoped %}{{ cell.source  }}
 {% endblock rawcell %}
 
 {% block unknowncell scoped %}
 unknown type  {{cell.type}}
 {% endblock unknowncell %}