Allowing azurerm_cdn_endpoint's to be updated

[tombuildsstuff]

Fixes a bug which meant CDN Endpoints couldn't be updated - also updated the CDN tests to match the new function format

Fixes #97

[tombuildsstuff]

Tests pass:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -timeout 300m -parallel 5 -run TestAccAzureRMCdn
=== RUN   TestAccAzureRMCdnEndpoint_importWithTags
--- PASS: TestAccAzureRMCdnEndpoint_importWithTags (209.73s)
=== RUN   TestAccAzureRMCdnProfile_importWithTags
--- PASS: TestAccAzureRMCdnProfile_importWithTags (142.53s)
=== RUN   TestAccAzureRMCdnEndpoint_basic
--- PASS: TestAccAzureRMCdnEndpoint_basic (189.55s)
=== RUN   TestAccAzureRMCdnEndpoint_disappears
--- PASS: TestAccAzureRMCdnEndpoint_disappears (331.28s)
=== RUN   TestAccAzureRMCdnEndpoint_updateHostHeader
--- PASS: TestAccAzureRMCdnEndpoint_updateHostHeader (209.12s)
=== RUN   TestAccAzureRMCdnEndpoint_withTags
--- PASS: TestAccAzureRMCdnEndpoint_withTags (197.86s)
=== RUN   TestAccAzureRMCdnProfile_basic
--- PASS: TestAccAzureRMCdnProfile_basic (129.44s)
=== RUN   TestAccAzureRMCdnProfile_withTags
--- PASS: TestAccAzureRMCdnProfile_withTags (136.37s)
=== RUN   TestAccAzureRMCdnProfile_NonStandardCasing
--- PASS: TestAccAzureRMCdnProfile_NonStandardCasing (153.47s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	1699.375s

[radeksimko]

LGTM 👍

[ghost]

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