One more fix for ensuring valid preview server custom headers

Author: daveaglick

src/Statiq.Web.Hosting/Middleware/CustomHeadersMiddleware.cs

@@ -34,7 +34,7 @@ public async Task InvokeAsync(HttpContext context)
 
             foreach (KeyValuePair<string, string> header in _customHeaders)
             {
-                context.Response.Headers.Append(header.Key, header.Value);
+                context.Response.Headers.Append(header.Key, header.Value ?? string.Empty);
             }
         }
     }

src/Statiq.Web/Commands/PreviewCommand.cs

@@ -224,17 +224,27 @@ internal static void GetKeyValues(
             }
         }
 
-        private static Dictionary<string, string> GetKeyValues(IEnumerable<string> keysAndValues, string optionName)
+        private static Dictionary<string, string> GetKeyValues(
+            IEnumerable<string> keysAndValues,
+            string optionName)
         {
             Dictionary<string, string> dictionary = new Dictionary<string, string>();
             foreach (string keyAndValue in keysAndValues)
             {
-                string[] split = keyAndValue.Split('=');
-                if (split.Length < 2)
+                int equalsLocation = keyAndValue.IndexOf("=");
+                if (equalsLocation == 0)
+                {
+                    throw new ArgumentException($"Invalid {optionName} (no key): {keyAndValue}");
+                }
+                if (equalsLocation >= keyAndValue.Length - 1)
+                {
+                    throw new ArgumentException($"Invalid {optionName} (no value): {keyAndValue}");
+                }
+                if (equalsLocation < 0)
                 {
-                    throw new ArgumentException($"Invalid {optionName} {keyAndValue} specified.");
+                    throw new ArgumentException($"Invalid {optionName} (no equals sign): {keyAndValue}");
                 }
-                dictionary[split[0].Trim().Trim('\"')] = split[1].Trim().Trim('\"');
+                dictionary[keyAndValue.Substring(0, equalsLocation)] = keyAndValue.Substring(equalsLocation + 1);
             }
             return dictionary;
         }