add in simple http-response example (#281)

* add in simple http-response example

* move to 57.0a1

http-response/README.md

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+# HTTP Response parser
+
+## What it does
+
+Listens to HTTP Responses from example.com and changes the body of the response as it comes through. So that the word "Example" on https://example.com becomes "WebExtension Example".
+
+## What it shows
+
+How to use the response parser on bytes.
+
+Icon is from: https://www.iconfinder.com/icons/763339/draw_edit_editor_pen_pencil_tool_write_icon#size=128

http-response/background.js

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+function listener(details) {
+  let filter = browser.webRequest.filterResponseData(details.requestId);
+  let decoder = new TextDecoder("utf-8");
+  let encoder = new TextEncoder();
+
+  filter.ondata = event => {
+    let str = decoder.decode(event.data, {stream: true});
+    // Just change any instance of Example in the HTTP response
+    // to WebExtension Example.
+    str = str.replace(/Example/g, 'WebExtension Example');
+    filter.write(encoder.encode(str));
+    filter.disconnect();
+  }
+
+  return {};
+}
+
+browser.webRequest.onBeforeRequest.addListener(
+  listener,
+  {urls: ["https://example.com/*"], types: ["main_frame"]},
+  ["blocking"]
+);

http-response/manifest.json

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+{
+
+  "description": "Altering HTTP responses",
+  "manifest_version": 2,
+  "name": "http-response-filter",
+  "version": "1.0",
+  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/http-response",
+  "icons": {
+    "48": "pen.svg"
+  },
+
+  "permissions": [
+    "webRequest", "webRequestBlocking", "https://example.com/*"
+  ],
+
+  "background": {
+    "scripts": ["background.js"]
+  },
+
+  "applications": {
+    "gecko": {
+      "strict_min_version": "57.0a1"
+    }
+  }
+
+}

http-response/pen.svg

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+<?xml version="1.0" ?><svg height="24px" version="1.1" viewBox="0 0 24 24" width="24px" xmlns="http://www.w3.org/2000/svg" xmlns:sketch="http://www.bohemiancoding.com/sketch/ns" xmlns:xlink="http://www.w3.org/1999/xlink"><title/><desc/><defs/><g fill="none" fill-rule="evenodd" id="miu" stroke="none" stroke-width="1"><g id="Artboard-1" transform="translate(-899.000000, -227.000000)"><g id="slice" transform="translate(215.000000, 119.000000)"/><path d="M914.000027,248.002414 L914.000464,232.002414 L914.000464,232.002414 L907.001354,232.002414 L907.000079,248.002414 L914.000027,248.002414 Z M913.998311,249.002414 L910.501672,254 L907.00169,249.002414 L913.998311,249.002414 Z M914.000492,231.002414 L914.000574,228.002414 C914.000574,227 912.99816,227 912.99816,227 L908.004086,227 C908.004086,227 907.001672,227 907.001672,228.002414 L907.001433,231.002414 L914.000492,231.002414 L914.000492,231.002414 Z" fill="#000000" id="editor-pencil-pen-edit-write-glyph" transform="translate(910.500326, 240.500000) rotate(45.000000) translate(-910.500326, -240.500000) "/></g></g></svg>