Unable to change product authentication method

[lvillen]

Which issue(s) this PR fixes

THREESCALE-8570 Unable to change product authentication method

Verification steps

1. Navigate to Product setting page (/apiconfig/services/{product_id}/settings).
2. Change authentication method.
3. Click Update Product button at the bottom of the page.
4. You get 'Product information updated' message but authentication method won't change.

[akostadinov]

🐚 rg -U "form.input[^=]+name:"
app/views/provider/signups/_user.html.slim
1:= form.inputs name: '', for: @user do |user|

app/views/provider/admin/user/personal_details/edit.html.slim
8:  = form.inputs name: 'User Information' do
14:    = form.inputs name: "Provide your current password and update your personal details" do

app/views/provider/admin/webhooks/_form.html.slim
1:= form.inputs name: 'Behaviour' do
8:= form.inputs name: 'Webhooks endpoint', class: 'inputs' do
17:= form.inputs name: 'Events that trigger webhooks', class: 'inputs' do
22:          = form.inputs name: 'Accounts' do
29:          = form.inputs name: 'Users' do
34:          = form.inputs name: 'Applications' do
45:          = form.inputs name: 'Keys' do

app/views/provider/admin/accounts/new.html.slim
4:  = form.inputs name: 'User Information', for: [:user, @user] do |user|
8:  = form.inputs name: 'Organization Information' do

app/views/buyers/accounts/edit.html.slim
12:    = form.inputs name: 'Contact Details', class: 'inputs' do

app/views/api/services/forms/_integration_settings.html.slim
18:    = form.input :proxy_authentication_method,
19:            name: 'service[backend_version]',