Extension update requests are sent in JSON are not handled correctly

[jumde]

From: brave/go-update#17

Description

Requests to update extensions are sent in JSON instead of XML. These requests are not handled correctly by go-updater.

Test Plan

See issue: brave/brave-core#1602

Steps to Reproduce

1. Run go-updater locally
2. Try updating extensions
3. See update request in JSON

Actual result:

Update request in JSON

Expected result:

Update request in XML

Reproduces how often:

Easily

Brave version (brave://version info)

master

[srirambv]

Verification passed on

Brave	0.60.40 Chromium: 72.0.3626.109 (Official Build) (64-bit)
Revision	fae8db7ab9280fa6704a59980263c804f809ebd5-refs/branch-heads/3626@{#857}
OS	Linux

• Verified test plan from Issue 3258: Using BraveUpdateClientConfig to send requests for extension update in XML and disable CUP signing brave-core#1602

Verification passed on

Brave	0.60.44 Chromium: 72.0.3626.109 (Official Build) (64-bit)
Revision	fae8db7ab9280fa6704a59980263c804f809ebd5-refs/branch-heads/3626@{#857}
OS	Windows 10

• Verified test plan from Issue 3258: Using BraveUpdateClientConfig to send requests for extension update in XML and disable CUP signing brave-core#1602

Before update:

After Update:

Verified passed with

Brave	0.60.44 Chromium: 72.0.3626.109 (Official Build) (64-bit)
Revision	fae8db7ab9280fa6704a59980263c804f809ebd5-refs/branch-heads/3626@{#857}
OS	Mac OS X

• Verified test plan from Issue 3258: Using BraveUpdateClientConfig to send requests for extension update in XML and disable CUP signing brave-core#1602 on a call with PJ
  Extensions before update:

Extensions after update: