…://github.com/kterada0509/terraform-provider-aws into kterada0509-feature/add-support-autoscaling-policy-import

…erraform state

Previously, the resource was silently failing to read this attribute and therefore unable to perform drift detection or properly import the resource (currently being added).

```
--- FAIL: TestAccAWSAutoscalingPolicy_basic (85.72s)
    testing.go:538: Step 2 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.

        (map[string]string) {
        }

        (map[string]string) (len=4) {
         (string) (len=17) "step_adjustment.#": (string) (len=1) "1",
         (string) (len=54) "step_adjustment.2042107634.metric_interval_lower_bound": (string) (len=1) "2",
         (string) (len=54) "step_adjustment.2042107634.metric_interval_upper_bound": (string) "",
         (string) (len=45) "step_adjustment.2042107634.scaling_adjustment": (string) (len=1) "1"
        }
```

Returning the error from `d.Set()`:

```
--- FAIL: TestAccAWSAutoscalingPolicy_basic (14.03s)
    testing.go:538: Step 0 error: Error applying: 1 error occurred:
        	* aws_autoscaling_policy.foobar_step: 1 error occurred:
        	* aws_autoscaling_policy.foobar_step: error setting step_adjustment: step_adjustment.0.metric_interval_lower_bound: '' expected type 'string', got unconvertible type 'float64'
```

Output from acceptance testing after adjustments:

```
--- PASS: TestAccAWSAutoscalingPolicy_zerovalue (44.61s)
--- PASS: TestAccAWSAutoscalingPolicy_SimpleScalingStepAdjustment (46.60s)
--- PASS: TestAccAWSAutoscalingPolicy_TargetTrack_Predefined (48.75s)
--- PASS: TestAccAWSAutoscalingPolicy_disappears (72.73s)
--- PASS: TestAccAWSAutoscalingPolicy_TargetTrack_Custom (74.47s)
--- PASS: TestAccAWSAutoscalingPolicy_upgrade (76.43s)
--- PASS: TestAccAWSAutoscalingPolicy_basic (82.25s)
```