page_rule: Set default value for cache_keys.host.resolved

[jacobbednarz]

Updates the cache_keys.host.resolved schema to include a default value
as the API expects it and without it, the resource panics.

Fixes #810

[jacobbednarz]

Integration tests are all passing

=== RUN   TestAccCloudflarePageRule_Import
=== PAUSE TestAccCloudflarePageRule_Import
=== RUN   TestAccCloudflarePageRule_ImportWithBrowserCacheTTL30
--- PASS: TestAccCloudflarePageRule_ImportWithBrowserCacheTTL30 (6.41s)
=== RUN   TestAccCloudflarePageRule_ImportWithoutBrowserCacheTTL
--- PASS: TestAccCloudflarePageRule_ImportWithoutBrowserCacheTTL (4.80s)
=== RUN   TestAccCloudflarePageRule_Basic
--- PASS: TestAccCloudflarePageRule_Basic (3.55s)
=== RUN   TestAccCloudflarePageRule_FullySpecified
--- PASS: TestAccCloudflarePageRule_FullySpecified (3.45s)
=== RUN   TestAccCloudflarePageRule_ForwardingOnly
--- PASS: TestAccCloudflarePageRule_ForwardingOnly (4.25s)
=== RUN   TestAccCloudflarePageRule_ForwardingAndOthers
--- PASS: TestAccCloudflarePageRule_ForwardingAndOthers (0.08s)
=== RUN   TestAccCloudflarePageRule_Updated
--- PASS: TestAccCloudflarePageRule_Updated (6.27s)
=== RUN   TestAccCloudflarePageRule_CreateAfterManualDestroy
--- PASS: TestAccCloudflarePageRule_CreateAfterManualDestroy (6.35s)
=== RUN   TestAccCloudflarePageRule_UpdatingZoneForcesNewResource
--- PASS: TestAccCloudflarePageRule_UpdatingZoneForcesNewResource (7.11s)
=== RUN   TestAccCloudflarePageRuleMinifyAction
--- PASS: TestAccCloudflarePageRuleMinifyAction (3.84s)
=== RUN   TestAccCloudflarePageRule_CreatesBrowserCacheTTLIntegerValues
--- PASS: TestAccCloudflarePageRule_CreatesBrowserCacheTTLIntegerValues (3.95s)
=== RUN   TestAccCloudflarePageRule_CreatesBrowserCacheTTLThatRespectsExistingHeaders
--- PASS: TestAccCloudflarePageRule_CreatesBrowserCacheTTLThatRespectsExistingHeaders (4.59s)
=== RUN   TestAccCloudflarePageRule_UpdatesBrowserCacheTTLToSameValue
--- PASS: TestAccCloudflarePageRule_UpdatesBrowserCacheTTLToSameValue (6.51s)
=== RUN   TestAccCloudflarePageRule_UpdatesBrowserCacheTTLThatRespectsExistingHeaders
--- PASS: TestAccCloudflarePageRule_UpdatesBrowserCacheTTLThatRespectsExistingHeaders (6.10s)
=== RUN   TestAccCloudflarePageRule_DeletesBrowserCacheTTLThatRespectsExistingHeaders
--- PASS: TestAccCloudflarePageRule_DeletesBrowserCacheTTLThatRespectsExistingHeaders (7.00s)
=== RUN   TestAccCloudflarePageRuleEdgeCacheTTLNotClobbered
--- PASS: TestAccCloudflarePageRuleEdgeCacheTTLNotClobbered (6.79s)
=== RUN   TestAccCloudflarePageRuleCacheKeyFieldsBasic
--- PASS: TestAccCloudflarePageRuleCacheKeyFieldsBasic (3.86s)
=== RUN   TestAccCloudflarePageRuleCacheKeyFields2
--- PASS: TestAccCloudflarePageRuleCacheKeyFields2 (3.68s)
=== RUN   TestAccCloudflarePageRuleCacheTTLByStatus
--- PASS: TestAccCloudflarePageRuleCacheTTLByStatus (4.10s)
=== CONT  TestAccCloudflarePageRule_Import
--- PASS: TestAccCloudflarePageRule_Import (5.36s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/cloudflare	98.088s