Add docs and tests case for transformHeader with index

docs/docs.html

@@ -508,7 +508,7 @@ var csv = Papa.unparse({
 									<code>transformHeader</code>
 								</td>
 								<td>
-									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument.<br>
+									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument and the index as second.<br>
 									Only available starting with version 5.0.
 								</td>
 							</tr>

tests/test-cases.js

@@ -767,6 +767,15 @@ var PARSE_TESTS = [
 			errors: []
 		}
 	},
+	{
+		description: "transformHeader accepts and optional index attribute",
+		input: 'A,B,C\r\na,b,c',
+		config: { header: true, transformHeader: function(header, i) { return i % 2 ? header.toLowerCase() : header; } },
+		expected: {
+			data: [{"A": "a", "b": "b", "C": "c"}],
+			errors: []
+		}
+	},
 	{
 		description: "Line ends with quoted field, first field of next line is empty using headers",
 		input: 'a,b,"c"\r\nd,e,"f"\r\n,"h","i"\r\n,"k","l"',