Add a way to get all keys of the headers. (#131)

Fixes #120.

src/headers.toit

@@ -23,6 +23,13 @@ class Headers:
 
   constructor.private_ .headers_:
 
+  /**
+  The keys of the header.
+  */
+  keys -> List:
+    if not headers_: return []
+    return headers_.keys
+
   /**
   Returns a single string value for the header or null if the header is not
     present.  If there are multiple values, the last value is returned.

tests/headers_test.toit

@@ -8,6 +8,7 @@ import http
 
 main:
   test_from_map
+  test_keys
 
 /**
 Converts the given $headers to a string.
@@ -35,3 +36,10 @@ test_from_map:
 
   headers = http.Headers.from_map {"foo": ["bar", "baz"], "Foo": "corge"}
   expect_equals "Foo: bar\r\nFoo: baz\r\nFoo: corge\r\n" (stringify headers)
+
+test_keys:
+  headers := http.Headers.from_map {"foo": ["bar", "baz"], "qux": "quux"}
+  expect_list_equals ["Foo", "Qux"] headers.keys
+
+  headers = http.Headers
+  expect_list_equals [] headers.keys