resource/azurerm_sql_database: Fix requested_service_objective_name updates

[bflad]

Related: #590 (comment)

Previously, when attempting to update an azurerm_sql_database resource requested_service_objective_name argument without specifying a request_service_objective_id, the Azure would return an error because the resource was sending both the old (computed) ID and the new (updated) name:

$ make testacc TEST=./azurerm TESTARGS='-run=TestAccAzureRMSqlDatabase_requestedServiceObjectiveName'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -run=TestAccAzureRMSqlDatabase_requestedServiceObjectiveName -timeout 180m
=== RUN   TestAccAzureRMSqlDatabase_requestedServiceObjectiveName
--- FAIL: TestAccAzureRMSqlDatabase_requestedServiceObjectiveName (195.12s)
	testing.go:513: Step 1 error: Error applying: 1 error(s) occurred:

		* azurerm_sql_database.test: 1 error(s) occurred:

		* azurerm_sql_database.test: Code="0" Message="The requested Service Objective Name does not match the requested Service Objective Id."
FAIL
FAIL	github.com/terraform-providers/terraform-provider-azurerm/azurerm	195.151s
make: *** [testacc] Error 1

This update allows the resource to successfully update:

$ make testacc TEST=./azurerm TESTARGS='-run=TestAccAzureRMSqlDatabase_requestedServiceObjectiveName'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -run=TestAccAzureRMSqlDatabase_requestedServiceObjectiveName -timeout 180m
=== RUN   TestAccAzureRMSqlDatabase_requestedServiceObjectiveName
--- PASS: TestAccAzureRMSqlDatabase_requestedServiceObjectiveName (282.32s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	282.352s

[katbyte]

@bflad,

Thank you for the PR, this LGTM! 👍

[ghost]

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