Fix external downloader when there is no http_header

youtube_dlc/downloader/external.py

@@ -115,8 +115,10 @@ class CurlFD(ExternalFD):
 
     def _make_cmd(self, tmpfilename, info_dict):
         cmd = [self.exe, '--location', '-o', tmpfilename]
+        if info_dict.get('http_headers') is not None:
             for key, val in info_dict['http_headers'].items():
                 cmd += ['--header', '%s: %s' % (key, val)]
+
         cmd += self._bool_option('--continue-at', 'continuedl', '-', '0')
         cmd += self._valueless_option('--silent', 'noprogress')
         cmd += self._valueless_option('--verbose', 'verbose')
@@ -150,6 +152,7 @@ class AxelFD(ExternalFD):
 
     def _make_cmd(self, tmpfilename, info_dict):
         cmd = [self.exe, '-o', tmpfilename]
+        if info_dict.get('http_headers') is not None:
             for key, val in info_dict['http_headers'].items():
                 cmd += ['-H', '%s: %s' % (key, val)]
         cmd += self._configuration_args()
@@ -162,6 +165,7 @@ class WgetFD(ExternalFD):
 
     def _make_cmd(self, tmpfilename, info_dict):
         cmd = [self.exe, '-O', tmpfilename, '-nv', '--no-cookies']
+        if info_dict.get('http_headers') is not None:
             for key, val in info_dict['http_headers'].items():
                 cmd += ['--header', '%s: %s' % (key, val)]
         cmd += self._option('--limit-rate', 'ratelimit')
@@ -189,6 +193,7 @@ class Aria2cFD(ExternalFD):
         if dn:
             cmd += ['--dir', dn]
         cmd += ['--out', os.path.basename(tmpfilename)]
+        if info_dict.get('http_headers') is not None:
             for key, val in info_dict['http_headers'].items():
                 cmd += ['--header', '%s: %s' % (key, val)]
         cmd += self._option('--interface', 'source_address')
@@ -206,6 +211,8 @@ class HttpieFD(ExternalFD):
 
     def _make_cmd(self, tmpfilename, info_dict):
         cmd = ['http', '--download', '--output', tmpfilename, info_dict['url']]
+
+        if info_dict.get('http_headers') is not None:
             for key, val in info_dict['http_headers'].items():
                 cmd += ['%s:%s' % (key, val)]
         return cmd
@@ -253,7 +260,7 @@ class FFmpegFD(ExternalFD):
         # if end_time:
         #     args += ['-t', compat_str(end_time - start_time)]
 
-        if info_dict['http_headers'] and re.match(r'^https?://', url):
+        if info_dict.get('http_headers') is not None and re.match(r'^https?://', url):
             # Trailing \r\n after each HTTP header is important to prevent warning from ffmpeg/avconv:
             # [http @ 00000000003d2fa0] No trailing CRLF found in HTTP header.
             headers = handle_youtubedl_headers(info_dict['http_headers'])