pastebin backend: Use a default expiration of one month

I don't want to clog pastebin's database with my unit tests!

weboob/backends/pastebin/browser.py

@@ -62,6 +62,7 @@ class PastebinBrowser(BaseBrowser):
     def api_post_paste(self, dev_key, paste):
         data = {'api_dev_key': dev_key,
                 'api_option': 'paste',
+                'api_paste_expire_date': '1M',
                 'api_paste_code': paste.contents.encode(self.ENCODING),
         }
         if paste.title:

weboob/backends/pastebin/pages.py

@@ -44,4 +44,5 @@ class PostPage(BasePage):
         self.browser.select_form(name='myform')
         self.browser['paste_code'] = paste.contents.encode(self.browser.ENCODING)
         self.browser['paste_name'] = paste.title.encode(self.browser.ENCODING)
+        self.browser['paste_expire_date'] = ['1M']
         self.browser.submit()